Coherent beam combining of high power broad- area laser diode ...

Coherent beam combining of high power broadarea laser diode array with near diffraction limited beam quality and high power conversion efficiency B. Liu1,2,* and Y. Braiman1,2 1

2

Center for Engineering Science Advanced Research, Computer Science & Mathematics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA Department of Mechanical, Aerospace, and Biomedical Engineering, University of Tennessee, Knoxville, Tennessee 37996, USA * [email protected]

Abstract: We explored a path of achieving high quality phase-locking of broad-area laser diode (BALD) array that operates at high electrical to optical power conversion efficiency (PCE). We found that (a) improving single transverse mode control for each individual BALD, (b) employing global Talbot optical coupling among diodes, and (c) enhancing strength of optical coupling among diodes are key factors in achieving high quality phase-locking of high power BALD array. Subsequently, we redesigned and improved a V-shaped external Talbot cavity and employed low reflectivity anti-reflection (AR) coated, low-“smile” BALD array to meet these three important requirements. We demonstrated near-diffraction limit far-field coherent pattern with 19% PCE and 95% visibility. The far-field angle (fullwidth at half-maximum (FWHM)) of center lobe was measured as 1.5 diffraction angular limited with visibility of 99% for 5A injection current and 1.6 diffraction angular limited with visibility of 95% for 14A injection current. Power scaling of diode array is discussed. ©2013 Optical Society of America OCIS codes: (140.2010) Diode laser arrays; (140.3298) Laser beam combining; (140.3300) Laser beam shaping; (140.3410) Laser resonators.

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#199939 - $15.00 USD Received 22 Oct 2013; revised 1 Dec 2013; accepted 5 Dec 2013; published 11 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031218 | OPTICS EXPRESS 31218

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#199939 - $15.00 USD Received 22 Oct 2013; revised 1 Dec 2013; accepted 5 Dec 2013; published 11 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031218 | OPTICS EXPRESS 31219

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1. Introduction Laser beam combining provides a path to simultaneously scale up power, increase brightness and reduce thermal burden in laser arrays [1]. Coherent beam combining of lasers is an appealing technology in designing high power coherent laser source. Coherent laser source can be useful in a variety of applications such as directed energy, free-space optical communication, and nonlinear optics processes including high-efficiency blue-green light generation. Locking phases of lasers is a key step to achieve coherent beam combining. Phase locking designs can be grouped into two categories: active phase locking and passive phase locking. The master-oscillator power amplifier (MOPA) design is extensively used in active phase locking approach. In MOPA design, the center wavelength of each individual amplifier channel is locked by optical injection from master oscillator. Phase adjustments of each channel are limited by the response bandwidth of feedback control electronics [2–7]. Passive phase locking employs optical coupling among lasers in either monolithic chip or external cavity. As a result, lasers oscillate in a collective coupled-mode pattern [8–21]. It was demonstrated that phases of chaotic lasers can be locked together due to optical coupling and optical coupling retains broad response bandwidth [22, 23]. Proper optical coupling with strong collective mode discrimination is the key for a successful passive phase locking. External Talbot cavity imposes diffraction feedback coupling with strong collective mode discrimination and therefore was successfully utilized for coherent beam combining of single mode laser diode (LD) array [8, 9, 11]. The combined power was limited by the number of phase locked LDs and the power of each LD. Increasing the power of an individual single LD is a plausible path to increase coherent output power. Two approaches (tapered laser diode and slab-coupled optical waveguide laser (SCOWL)) were implemented to increase the output power of single LD with well-defined transverse mode. Coherent beam combining of a tapered LD array and SCOWL array with external Talbot cavity was investigated in references [17–19].

#199939 - $15.00 USD Received 22 Oct 2013; revised 1 Dec 2013; accepted 5 Dec 2013; published 11 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031218 | OPTICS EXPRESS 31220

Commercial broad-area laser diode (BALD) emits high optical power with high PCE ranged from 50% to 70% [24, 25]. The cost of BALD is low since the fabrication of BALD is relatively simple. However, wide emitter size reduces transverse mode discrimination and low mode discrimination results in multi-transverse mode oscillations and consequently low beam quality. Limiting the number of BALD transverse modes constitutes a path of increasing the beam quality of BALD. Applying low anti-reflection (AR) coating to a BALD and employing external optical feedback to a BALD can limit the number of transverse modes. A variety of external feedback designs can limit the number of transverse modes of BALD and improve the beam quality [26–38]. The off-axis optical feedback was introduced to enhance one transverse mode and significantly increase beam quality of BALD [33, 34, 36, 38]. Recently, the off-axis external cavity and Talbot external cavity were combined together to form a V-shaped external Talbot cavity [39, 40]. Coherent beam combining from an array consisting of 47 BALDs was demonstrated. Such array emitted 12.8 Watts output power [40]. However the PCE of coherent beam combining array was in the range of 12% and the farfield angular distribution was a couple of times larger than diffraction limited. This PCE was lower than the values demonstrated for coherent beam combination of tapered LD array external Talbot cavity (estimated ~18%) and SCOWL array external Talbot cavity (estimated ~22%) [18, 19]. Near diffraction angular limit far-field was demonstrated with tapered LD array external Talbot cavity [18]. An important question can be asked: is it possible to design an external cavity that make possible high PCE operation (in the range of 20%) and near diffraction limit far field emission from a BALD array? In this paper, we explored a path of achieving high quality passive phase-locking of highpower BALD array with near diffraction angular limit far-field and high PCE. We found that (a) improving single transverse mode control for each individual BALD on array, (b) implementing global Talbot optical coupling among diodes, and (c) enhancing strength of optical coupling among diodes are key factors in achieving high quality phase-lock high power BALD. To meet these three important requirements, we redesigned and improved a Vshaped external Talbot cavity and employed low reflectivity anti-reflection (AR) coated, low“smile” BALD array. Low reflectivity AR-coating allows external cavity to have better mode selection. Design-improved V-shaped external Talbot cavity (a telescope along fast-axis matched with fast-axis-collimation (FAC) (NA = 0.8)) narrows spectral line-width and reduces the “smile effect”. Low-“smile” BALD array subject to design-improved V-shaped external Talbot cavity allows sufficient optical coupling among diodes. Therefore the designimproved V-shaped external Talbot cavity increases the diffraction coupling among diodes. We implemented design-improved V-shaped external Talbot cavity with an array of 10 BALDs and demonstrated PCE in the range of 19% while maintaining coherent beam with well-maintained coherence (95% visibility). The near-diffraction angular limited far-field pattern was demonstrated and compared with the numerical simulation of far-field pattern from in-phase locked 10 LDs. The far-field angle (full-width at half-maximum (FWHM)) of center lobe was measured as 1.5 diffraction angular limited with visibility of 99% for 5 A injection current and 1.6 diffraction angular limited with visibility of 95% for 14 A injection current. 2. Experiment PCE of BALD with single transverse mode emission ranges from approximately 9.5% (M2~1.03) to 17.6% (M2~2.57) [33]. With diffractive grating loaded external cavity, the demonstrated PCE was even lower and was estimated to be approximately 5% (200 mW, 2A) with M2~1.3 and laser spectrum line-width ~8 pm [34]. In order to suppress the intrinsic BALD mode oscillation and increase PCE of the single transverse mode emission, the ARcoating with lower reflectivity is needed. High PCE (estimated as ~17%) was achieved with high quality beam (M2~1.3) and narrow spectrum line-width (~2 MHz) using high-quality AR-coated (R